This invention relates to an optical composite material and a method for its production.
The requirements for optical materials which are used in microlithography projection exposure apparatuses are generally very high. These materials must on the one hand withstand high laser intensity in the UV or VUV wavelength range and on the other hand also have a sufficiently large transmission in this wavelength range.
In immersion systems, the requirements for the optical terminating element, which is in contact with the immersion liquid, are particularly high, since it should have a maximum refractive index for generating a large numerical aperture on the one hand and on the other hand withstand the relatively high energy density in the region close to the wafer.
The terminating element usually consists either of crystalline materials (e.g. CaF2) or amorphous materials such as e.g. quartz glass (fused silica). The advantage of crystalline materials is that they have a high radiation resistance, but their production with the required purity and optical quality requires great expense. The selection of suitable materials is moreover limited by the predetermined material properties of the crystals. In particular, there are only very few crystalline materials which meet both the requirements for high UV transmission and increased refraction index at the same time. On the other hand, quartz glass (fused silica) has already a relatively high refractive index but, since it is an amorphous material, it is not as stable as highly-regular crystalline materials. Moreover, systems with a numerical aperture of more than 1.4 require a material having a higher refractive index than quartz glass (1.56), since this refractive index should be adjusted to the high refractive index of the immersion liquid.